1. Field of the Invention
The present invention relates to a method for manufacturing a rare earth magnet and a powder compacting apparatus used in the manufacturing method.
2. Discussion of the Related Art
A rare earth sintered magnet is produced by pulverizing an alloy for a rare earth magnet to form alloy powder, compacting the alloy powder, and subjecting the alloy powder to a sintering process and an aging process. Currently, there are two kinds of magnets known as the rare earth sintered magnets, i.e., a samarium-cobalt magnet and a neodymium-iron-boron magnet each of which are widely used in various fields. Hereinafter, the magnet of neodymium, iron, and boron system is referred to as an xe2x80x9cR-T-(M)-B type magnetxe2x80x9d, where R represents a rare earth element or yttrium, T represents iron, or a transition metal in which cobalt or nickel is substituted for part of iron, M represents an additional element, and B represents boron or a compound of boron and carbon. Between the two kinds of magnets, the R-T-(M)-B type magnet exhibits the maximum magnetic energy product among various kinds of magnets, and the price thereof is relatively cheap. For these reasons, the R-T-(M)-B type magnet is used for various kinds of electronics appliances.
When an anisotropic rare earth sintered magnet is manufactured, an orienting magnetic field is applied to magnetic powder during press compaction. Thus, the produced compact is in a strongly magnetized condition. In order to remove the magnetization, a demagnetizing process is performed in a press, however, it is extremely difficult to attain the perfect demagnetization. Therefore, when the demagnetized compact is ejected from a die hole (a cavity) of a press, magnetic powder, which is dispersed around the die hole, is strongly attracted to the compact. According to measurements, a magnetization of 0.002 to 0.006 T (tesla) remains in the compact after the demagnetizing process.
Since the demagnetizing process is performed for the compact while in the cavity, the intensity variation of the magnetic field formed for the demagnetization process is designed so as to have the most suitable profile for the demagnetization of the compact in the center portion of the cavity. As a result, magnetic powder adhering to magnetic material components of a magnetic field generating portions positioned over and under the cavity and magnetic powder adhering on the die of the press and the like are demagnetized only a little. According to measurements, a magnetization of about 0.005 to 0.010 T remains in the powder adhering to a pole piece (a magnetic portion of an upper punch) accompanied with the magnetic field generating coil.
The compact and powder, both of which are magnetized, mutually attract each other strongly. Accordingly, when the compact is ejected from the cavity of the press and placed onto a carrying device, the magnetic powder adhering to the upper punch of the pressing apparatus and the magnetic powder scattered on the die are attracted to the compact, and firmly adsorbed to the surface of the compact.
In order to remove the magnetic powder adhering to the surface of the compact from the surface of the compact, nitrogen gas (N2 gas) was sprayed on the compact while the compact is being carried on a carrying belt and transported.
However, it is impossible to entirely remove the magnetic powder adhering to a portion of the compact which little receives the N2 gas. Therefore, the magnetic powder attracted to the surface of the compact by the strong magnetic force, results in the remaining magnetic powder being welded to the surface of a sintered compact body by sintering. This magnetic powder, welded by the sintering, increases the degree of unevenness in the surface of the sintered compact body. Thus, it is necessary to remove the welded portions by grinding to provide a smooth surface on the sintered body.
Conventionally, after the large block-like sintered, compact body was produced, the body was processed by cutting, so as to obtain a plurality of relatively small sintered bodies. In this instance, even if protrusions caused by the adhering powder existed on the surface of the sintered compact body, the protrusions in the surface of the respective sintered bodies cut out by the cutting process did not cause serious problems.
In order to improve the production yield of small magnets, however, a pressing process has been recently adopted in which the compact produced has the shape of the final product. In this instance, if the undesired magnetic powder adheres to the surface of the compact produced, the period of time to complete the grinding process after the sintering is increased, and the advantages of mass production are diminished.
Japanese Laid-Open Patent Publication No. 3-234603 discloses closes a powder removing device in which a ceramic powder compact situated in a cylindrical brush, and the powder adhering to the surface of the compact is blown off while the brush is rotating.
If these techniques are adapted to the production of a compact from rare earth magnetic powder, the following problems arise.
A compact of a rare earth alloy powder, in which the powder orientation in the magnetic field is significant, has a compact density that is suppressed to be as low as a density of 3.9 to 5.0 g/cm3, which is soft. Further, in the case where the rare earth alloy powder is produced by a rapid cooling method, the particle size distribution curve of the powder is sharp. Thus, the strength of the rare earth compact is lowered when compared with the strength of a compact using powder produced by an ingot casting method. Additionally, if the surface of the compact is rubbed with a brush, the corners of the compact may be lost, or the compact may be broken.
It takes time and effort to insert the rare earth compact into a powder removing device and to take it out of the powder removing device, such that the overall production yield is reduced.
An additional disadvantage, is that the recovered powder reacts with oxygen in the air, so as to be rapidly oxidized. Thus, the possibility exists that a burning accident may occur in the powder removing device which is a dangerous situation.
For the reasons described above, an optimum powder removing device is required in a method for manufacturing a rare earth sintered magnet.
A primary object of the present invention is to provide a method for manufacturing a rare earth magnet with increased quantity production characteristics in which method the undesired magnetic powder adsorbed on the surface of a compact is appropriately removed without breaking the compact, thereby reducing the period of time required for grinding a magnet after sintering.
Another object of the present invention is to provide a powder compacting apparatus suitably used in the above-mentioned manufacturing method.
The present invention relates to a method for manufacturing a rare earth magnet including:
a first step of producing a compact by compacting rare earth alloy powder in a predetermined space in an orienting magnetic field;
a second step of performing a demagnetizing process for the compact;
a third step of ejecting the compact from the predetermined space; and
a fourth step of performing a demagnetizing process for magnetic powder adhering to a surface of the compact by applying an additional magnetic field to the compact after the third step.
In a preferred embodiment, in the first step, the rare earth alloy powder is carried on a member disposed around the predetermined space in a condition where the alloy powder is in contact with the member which is fed into the predetermined space. In another preferred embodiment, the first step includes a step of compressing the rare earth alloy powder in a direction substantially identical to a direction in which the orienting magnetic field is applied to the rare earth alloy powder. In still another preferred embodiment, the magnetic powder adhering to the surface of the compact is magnetized by the orienting magnetic field in the first step. While in still another preferred embodiment, the magnetic powder is magnetized in a condition where the magnetic powder adheres to a magnetic portion included in means for applying the orienting magnetic field to the rare earth alloy powder. In another embodiment, after the third step, the magnetization of the compact. In another embodiment, the fourth step includes a step of applying an alternative magnetic field to the compact.
Preferably, the fourth step includes a step of applying a decremental alternating magnetic field to the compact while the compact is moving.
Preferably, the step of applying the decremental alternating magnetic field is performed by using a plurality of coils.
Preferably, the alternating magnetic field is configured by two or more pulse magnetic fields of different directions. In another preferred embodiment, the fourth step is performed by a plurality of coils, and magnetic fields respectively formed by the plurality of coils are reapplied to the compact while the compact is moving. In still another preferred embodiment, the maximum value of the additional magnetic filed in the vicinity of the surface of the compact is in the range of not less than 0.02 tesla nor greater than 0.5 tesla. Further, in a preferred embodiment, the fourth step includes a step of spraying a gas to the surface of the compact, which is preferably an inert gas.
The method may further include a step of placing the compact on a sintering base plate, wherein the demagnetizing process in the fourth step is performed enroute while moving the compact onto the sintering base plate from a position in which the compacting is performed.
The method may further include a step of recognizing a shape of the compact before the step of placing the compact on the sintering base plate, wherein the demagnetizing process of the fourth step is performed before the step of recognizing the shape of the compact.
The method may further include:
a step of placing the compact on a nonmagnetic mesh member for moving the compact from a first position to a second position;
a step of moving the compact on the nonmagnetic mesh member onto a sintering base plate in the second position; and
a step of sintering the compact, wherein the fourth step is performed between the first position and the second position.
In this embodiment, an additional magnetic field is formed by using an electromagnet disposed under the nonmagnetic mesh member. In a preferred embodiment, a suction port of a gas suction device is disposed under the mesh member, and magnetic powder removed from the surface of the compact is accommodated in the suction device. Preferably, the suctioned magnetic powder is isolated from the air.
Preferably, the fourth step is performed while the compact is moving on the nonmagnetic mesh member.
The method may further include a step of performing image processing by imaging the compact in the second position with an imaging device disposed on one side of the nonmagnetic mesh member and a light source disposed on the other side of the nonmagnetic mesh member.
In a preferred embodiment, the third step includes a step of ejecting the compact from the predetermined space by adsorbing the compact due to a magnetic force.
In a preferred embodiment, the rare earth alloy powder is powder of R-T-(M)-B type rare earth magnet alloy.
In a preferred embodiment, a lubricant is added to the rare earth alloy powder.
In a preferred embodiment, a density of the compact is in the range of 3.9 g/cm3 to 5.0 g/cm3.
In a preferred embodiment, the rare earth alloy powder is produced by a rapid cooling method.
In a preferred embodiment, the number of particles, having a particle diameter of 1.0 xcexcm or less, in the rare earth alloy powder is adjusted to be 10% or less of the entire number of particles in the rare earth alloy powder.
Alternatively, the powder compacting apparatus of the present invention includes:
a device for producing a compact by compacting rare earth alloy powder in an orienting magnetic field;
a device for performing a demagnetizing process for the compact; and
a device for performing a demagnetizing process for magnetic powder adhering to a surface of the compact by applying an additional magnetic field to the compact along the route for moving the compact from a position in which the compaction of the rare earth alloy powder is performed.
In a preferred embodiment, the device for producing the compact includes a magnetic field generator for generating the orienting magnetic field in a first direction, and a compacting device for compressing the rare earth alloy powder in the first direction.
In a preferred embodiment, the device for performing the demagnetizing process for the magnetic powder can apply an alternating magnetic field to the compact.
The powder compacting apparatus may further include a device for moving the compact, wherein the device for performing the demagnetizing process applies a decremental alternating magnetic field to the compact while the compact is moving.
Preferably, the device for performing the demagnetizing process for the magnetic powder includes a plurality of coils disposed along a route for moving the compact.
The powder compacting apparatus may further include a device for moving the compact, wherein the device for performing the demagnetizing process for the magnetic powder includes a plurality of coils disposed along a route for moving the compact, and while the compact is moving, the means for performing the demagnetizing process for the magnetic powder applies magnetic to the compact.
The powder compacting apparatus may further include a device for spraying a gas on the surface of the compact along the route for moving the compact from a position in which the compaction of the rare earth alloy powder is performed.
The powder compacting apparatus may further include a gas suction device having a suction port, wherein magnetic powder removed from the surface of the compact is drawn into the suction device.
The powder compacting apparatus may further include:
a nonmagnetic mesh member for moving the compact from a first position to a second position;
a device for placing the compact onto the nonmagnetic mesh member;
a device for driving the nonmagnetic mesh member; and
a device for moving the compact on the nonmagnetic mesh member onto a sintering base plate in the second position.
In a preferred embodiment, at least part of the device for performing the demagnetizing process for the magnetic powder is configured by an electromagnet disposed under the nonmagnetic mesh member.